1. Field of the Invention
The present invention relates to a radar device obtaining an optimum relative velocity resolution or distance resolution by changing a measurement time period of a measurement data in each range gate to calculate a distance to a target and a relative velocity.
2. Description of the Related Art
As one of radar devices that are mounted on vehicles, and used in an adaptive cruise control (ACC), a collision damping braking device or the like, a radar device of FMCW (Frequency Modulated Continuous Wave) system (hereinafter, referred to as a FMCW radar device) capable of detecting a distance to a target and a relative velocity at the same time has been known.
Incidentally, the FMCW system is one of transmission systems of the radar device. In this FMCW system, the distance to a target or the velocity can be obtained by calculating a difference in frequency between a transmission wave and a received wave (a reflected wave to be obtained by the transmission wave being reflected at the target).
In this FMCW radar, a transmission signal of CW (Continuous Wave) radar is subjected to FM modulation. An arrangement of the conventional FMCW radar is illustrated in FIG. 22.
FIGS. 23(a) and (b) are graphic diagrams for explaining an operation principle of the conventional FMCW radar. FIG. 23(a) illustrates a case where the relative velocity with respect to a target is 0, while FIG. 23(b) illustrates another case where the relative velocity is present.
When the frequency of an oscillator 312 is modulated with a triangular wave from a modulated signal generator 311 and radiated to the outside from a transmission antenna 314, the relationship between the frequency of the transmission signal and time is as indicated by the solid line in FIG. 23.
Whereas, the signal, being the transmission signal reflected at the target and received by a receiving antenna 315 undergoes a time delay due to the distance and a frequency deviation corresponding to the relative velocity.
When the relative velocity with respect to the target is 0, the signal (received wave) to be received at the receiving antenna 315 undergoes only the time delay, to be as indicated by the dot line in FIG. 23(a).
In the case where the relative velocity is present, as indicated by the dot line in FIG. 23(b), the signal (received wave) that is received at the receiving antenna 315 is superimposed with a Doppler frequency due to the movement of the target (FIG. 23(b) illustrates the case where the target approaches).
Incidentally, in FIG. 23, “τ” is a time difference between a transmission wave and a reflected wave, “ΔF” is a modulation width, “Tm” is a measurement time period, and “fm” is a modulated frequency.
As illustrated in FIG. 22, when this signal (that is, the signal, being the transmission signal reflected at the target and received by the receiving antenna 315) is mixed by a mixer 316 with a part of the transmission signal having been fetched out through a directional coupler 313, a beat signal can be obtained. Incidentally, reference numeral 317 designates an amplifier to amplify the beat signal from the mixer 316.
In case where the target moves, the beat frequency, in each modulation cycle (that is, in respective zones of the later-described “up phase” and “down phase”), is changed as expressed by the following equations (1) and (2).U=fr−fd   (1)D=Fr+fd   (2)where: U is a beat frequency in a frequency up zone (up phase), D is a beat frequency in a frequency down zone (down phase), fr is a beat frequency when the relative velocity is 0, and fd is a Doppler frequency based on the relative velocity.
Accordingly, in case where the beat signals U and D are measured in each cycle of modulation, the distance R to the target and the relative velocity V can be obtained as expressed by the following equations (3) and (4), fr, fd.Fr=(U+D)/2(∝R)   (3)Fd=(D−U)/2(∝V)   (4)
In this connection, a specific way of calculating the distance R to the target and the relative velocity V is now described.
As expressed by the equations (1) and (2), in the beat frequency (U) that is observed in the up phase and the beat frequency (D) that is observed in the down phase, there are included both the frequency fr to be determined by the distance R to the target and the frequency fd to be determined by the relative velocity of the target, and the equations (1) and (2) can be thought to be simultaneous equations including unknowns fr and fd.
Thus, with the beat frequencies U and D having been observed, fr and fd can be obtained, and the distance R and the relative velocity V can be obtained by the below-mentioned equations from these fr and fd.R=((c/4)×ΔF×fm)×fr V=(c/2f0)×fd where: c is a radio wave propagation speed (3×108 m/s), and f0 is a carrier frequency.
Incidentally, the above-mentioned technique is a generally known in the FMCW radar device, and corresponds to a publicly well-known technique, so that any particular prior art document is not disclosed herein.
In the above-described known FMCW radar device, however, in the case where a plurality of targets is present, a beat signal is generated with respect to each of the plurality of targets, thus making it hard to know a correspondence relationship between each beat signal and each target.
To meet this, a radar device has been disclosed and in which by changing a modulated frequency (fm) based on the distance to a target, the relative velocity of the target can be detected with high accuracy in the short distance, as well as it becomes easy to determine the correspondence between a plurality of targets and beat signals in the long distance (for example, see the Japanese Patent Publication (unexamined) No. 211145/1996)
That is, it is disclosed in the Japanese Patent Publication (unexamined) No. 211145/1996 that the modulated frequency (fm) is changed based on the distance to the target. More specifically, it is described such that “when a target is present in the short distance, by making fm smaller to make the relative velocity frequency relatively large, the relative velocity of the target can be detected with high accuracy; and when the target is present in the long distance, by making fm larger to make the relative velocity frequency relatively small, it comes to be easy to determine the correspondence between plurality of targets and beat signals”.
In the mentioned Japanese Patent Publication (unexamined) No. 211145/1996, however, the effective use of the radar device is achieved only on condition that the distance measurement results by means of the radar is correct. In the case where the beat signal is erroneously treated, and any error is made in the calculation of distance and relative velocity, setting of the modulated frequency (fm) will not be optimal.
That is, to correctly calculate the distance and the relative velocity of a target from the mentioned equations (3) and (4), the beat frequency U that is observed in the up phase and the beat frequency D that is measured in the down phase both need to be the beat frequency that is generated from the reflected wave from the same target. However, in the case where there is a plurality of targets, a plurality of beat frequencies will be observed in each phase.
Therefore, supposing that the combined U and D are of the same target, the distance and the relative velocity can be correctly calculated. Supposing that, however, any error is made in the combination of U and D, false distance and relative velocity will be calculated, and thus a problem exists in that setting of the modulated frequency (fm) is not optimal.
Moreover, in the case where the beam width of a radio wave to be transmitted from the transmission antenna is rather large, there are some cases in which both the target in the short distance and the target in the long distance are detected at the same time. In the case where the modulated frequency (fm) for the short distance is set, as to a target in the long distance, another problem exits in that the correspondence relationship of a beat frequency between in the up phase and in the down phase is less likely to determine.